Re: [PATCH 2/2] schemas: i2c: Introduce I2C bus extensions

Ayush Singh
2025-04-29T18:04:19Z

On 4/1/25 13:40, Herve Codina wrote:

> An I2C bus can be wired to the connector and allows an add-on board to
> connect additional I2C devices to this bus.
>
> Those additional I2C devices could be described as sub-nodes of the I2C
> bus controller node however for hotplug connectors described via device
> tree overlays there is additional level of indirection, which is needed
> to decouple the overlay and the base tree:
>
> --- base device tree ---
>
> i2c1: i2c@abcd0000 {
> compatible = "xyz,i2c-ctrl";
> i2c-bus-extension@0 {
> i2c-bus = <&i2c_ctrl>;
> };
> ...
> };
>
> i2c5: i2c@cafe0000 {
> compatible = "xyz,i2c-ctrl";
> i2c-bus-extension@0 {
> i2c-bus = <&i2c-sensors>;
> };
> ...
> };
>
> connector {
> i2c_ctrl: i2c-ctrl {
> i2c-parent = <&i2c1>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
>
> i2c-sensors {
> i2c-parent = <&i2c5>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
> };
>
> --- device tree overlay ---
>
> ...
> // This node will overlay on the i2c-ctrl node of the base tree
> i2c-ctrl {
> eeprom@50 { compatible = "atmel,24c64"; ... };
> };
> ...
>
> --- resulting device tree ---
>
> i2c1: i2c@abcd0000 {
> compatible = "xyz,i2c-ctrl";
> i2c-bus-extension@0 {
> i2c-bus = <&i2c_ctrl>;
> };
> ...
> };
>
> i2c5: i2c@cafe0000 {
> compatible = "xyz,i2c-ctrl";
> i2c-bus-extension@0 {
> i2c-bus = <&i2c-sensors>;
> };
> ...
> };
>
> connector {
> i2c-ctrl {
> i2c-parent = <&i2c1>;
> #address-cells = <1>;
> #size-cells = <0>;
>
> eeprom@50 { compatible = "atmel,24c64"; ... };
> };
>
> i2c-sensors {
> i2c-parent = <&i2c5>;
> #address-cells = <1>;
> #size-cells = <0>;
> };
> };
>
> Here i2c-ctrl (same goes for i2c-sensors) represent the part of I2C bus
> that is on the hot-pluggable add-on. On hot-plugging it will physically
> connect to the I2C adapter on the base board. Let's call the 'i2c-ctrl'
> node an "extension node".
>
> In order to decouple the overlay from the base tree, the I2C adapter
> (i2c@abcd0000) and the extension node (i2c-ctrl) are separate nodes.
>
> The extension node is linked to the I2C bus controller in two ways. The
> first one with the i2c-bus-extension available in I2C controller
> sub-node and the second one with the i2c-parent property available in
> the extension node itself.
>
> The purpose of those two links is to provide the link in both direction
> from the I2C controller to the I2C extension and from the I2C extension
> to the I2C controller.
>
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---
> dtschema/schemas/i2c/i2c-controller.yaml | 67 ++++++++++++++++++++++++
> 1 file changed, 67 insertions(+)
>
> diff --git a/dtschema/schemas/i2c/i2c-controller.yaml b/dtschema/schemas/i2c/i2c-controller.yaml
> index 018d266..509b581 100644
> --- a/dtschema/schemas/i2c/i2c-controller.yaml
> +++ b/dtschema/schemas/i2c/i2c-controller.yaml
> @@ -30,6 +30,13 @@ properties:
> minimum: 1
> maximum: 5000000
>
> + i2c-parent:
> + $ref: /schemas/types.yaml#/definitions/phandle
> + description:
> + In case of an I2C bus extension, reference to the I2C bus controller
> + this extension is connected to. In other word, reference the I2C bus
> + controller on the fixed side that drives the bus extension.
> +
> i2c-scl-falling-time-ns:
> description:
> Number of nanoseconds the SCL signal takes to fall; t(f) in the I2C
> @@ -159,6 +166,25 @@ allOf:
> - i2c-scl-has-clk-low-timeout
>
> patternProperties:
> + 'i2c-bus-extension@[0-9a-f]+$':
> + type: object
> + description:
> + An I2C bus extension connected to an I2C bus. Those extensions allow to
> + decouple I2C busses when they are wired to connectors.
> +
> + properties:
> + reg:
> + maxItems: 1
> +
> + i2c-bus:
> + $ref: /schemas/types.yaml#/definitions/phandle
> + description:
> + Reference to the extension bus.
> +
> + required:
> + - reg
> + - i2c-bus
> +
> '@[0-9a-f]+$':
> type: object
>
> @@ -221,3 +247,44 @@ dependentRequired:
> i2c-digital-filter-width-ns: [ i2c-digital-filter ]
>
> additionalProperties: true
> +
> +examples:
> + # I2C bus extension example involving an I2C bus controller and a connector.
> + #
> + # +--------------+ +-------------+ +-------------+
> + # | i2c@abcd0000 | | Connector | | Addon board |
> + # | (i2c1) +-----+ (i2c-addon) +-----+ (device@10) |
> + # | | | | | |
> + # +--------------+ +-------------+ +-------------+
> + #
> + # The i2c1 I2C bus is wired from a I2C controller to a connector. It is
> + # identified at connector level as i2c-addon bus.
> + # An addon board can be connected to this connector and connects a device
> + # (device@10) to this i2c-addon extension bus.
> + - |
> + i2c1: i2c@abcd0000 {
> + compatible = "xyz,i2c-ctrl";
> + reg = <0xabcd0000 0x100>;
> + #address-cells = <1>;
> + #size-cells = <0>;
> +
> + i2c-bus-extension@0 {
> + reg = <0>;
> + i2c-bus = <&i2c_addon>;
> + };
> + };
> +
> + connector {
> + i2c_addon: i2c-addon {
> + i2c-parent = <&i2c1>;
> + #address-cells = <1>;
> + #size-cells = <0>;
> +
> + device@10 {
> + compatible = "xyz,foo";
> + reg = <0x10>;
> + };
> + };
> + };
> +
> +...

Reviewed-by: Ayush Singh <ayush@beagleboard.org>